New data on kimberlite magmatism in southwestern Angola

First data on the geologic and geochemical compositions of kimberlites from nine kimberlite pipes of southwestern Angola are presented. In the north of the study area, there are the Chikolongo and Chicuatite kimberlite pipes; in the south, a bunch of four Galange pipes (I–IV); and in the central part, the Ochinjau, Palue, and Viniaty pipes. By geochemical parameters, these rocks are referred to as classical kimberlites: They bear mantle inclusions of ultrabasites, eclogites, various barophilic minerals (including ones of diamond facies), and diamonds. The kimberlite pipes are composed of petrographically diverse rocks: tuffstones, tuff breccias, kimberlite breccias, autolithic kimberlite breccias, and massive porphyritic kimberlites. In mineralogical, petrographic, and geochemical compositions the studied kimberlites are most similar to group I kimberlites of South Africa and Fe-Ti-kimberlites of the Arkhangel’sk diamondiferous province. Comparison of the mineralogical compositions of kimberlites from southwestern Angola showed that the portion of mantle (including diamondiferous) material of depth facies in kimberlite pipes regularly increases in the S-N direction. The northern diamond-bearing kimberlite pipes are localized in large destructive zones of NE strike, and the central and southern diamond-free pipes, in faults of N-S strike.     

Kimberlites distribution in Angola and prospective areas for new discoveries

Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.

     

Characteristics of the Geotectonics in South China and Their Constraints on Primary Diamond

The Asian continent is one of the best places in the world to study continental dynamics. In this region the tectonic framework of the South China plate is related to the activity of the Pacific plate and Indian plate since the Mesozoic. In the South China plate, as the ophiolitic melange of the middle Proterozoic in the Shaoxing-Yingtan-Tengxian-Beihai faulted zone was confirmed to be a subducting ocean, the evolution of the Yangtze block and Cathaysian block, which are located on both sides of the fault zone, becomes clearer and clearer. A primary diamond deposit, which was first found by Bao Chaomin and his colleagues in Longyou County of Zhejiang Province in 1998, originated from kimberlitoid pipes. The pipes are located in the Cretaceous basin beside this deep fault and the diamond-bearing pipe was formed deep within a favourable geological structure. Diamond there occurs as octahedral crystals without visual impurities. There are more than 100 similar pipes in the area, which have been poorly stud     

The Catoca kimberlite pipe, Republic of Angola: A paleovolcanological model

The Catoca kimberlite pipe is among the world’s largest primary diamond deposits. The Catoca volcanic edifice is only slightly eroded. Kimberlitic rocks of various facies compose a crater of about 1 km in diameter and a diatreme. The structure of the pipe and mining conditions of the deposit are complicated by intense intrapipe tectonic processes related to large-amplitude subsidence. Based on geological data, we propose a structural model of the deposit and a paleovolcanological model of the Catoca pipe formed during a full cycle beginning with a stage of active volcanism and completed by stages of gradually waning volcanic activity and sedimentation. It is suggested that the banded tuffisitic kimberlite of the crater zone was deposited at the stage of active volcanic eruption from specific pyroclastic suspension as a low-viscosity mixture of crystals and aqueous sol rich in serpentine.     

Distinctive structural features of magmatic complexes of the central type associated with ring faults

On the basis of a study of a large quantity of deep-seated xenoliths from the kimberlites of the Malo-Botuobuya, Daldyn-Alakit, Upper Muna, and Lower Olenek regions of Yakutia, we have discussed the distribution of the ultrabasic rocks and eclogites in the kimberlite pipes both on the basis of petrographic composition, and also on depth facies, and a comparison is presented of the mineral composition of the deep-seated inclusions and of the amounts of defined types of xenoliths with the diamond capacity of the kimberlites. The conclusion has been reached that: 1. the amounts of inclusions of deep-seated rocks vary significantly not only in kimberlites from the various diamond fields, but also in the pipes of a single diamond-bearing region; 2. the composition of the ultrabasic rocks and eclogites of the diamond-bearing pipes is distinguished from that of the inclusions of the non-diamond kimberlites in these rocks; and 3. the diamond capacity of the kimberlites has been determined by the depth of occurrence of the magmatic focus and the velocity of uprise (intrusion) of the melt during the formation of the kimberlitic diatremes —Authors.     

A rift model for the sedimentary diamond deposits of Brazil

Although several hundred kimberlites have been discovered in the past 20 years in Brazil, the Brazilian diamond production has been derived almost entirely from sedimentary deposits: diamond-bearing conglomerates of different ages and recent alluvials or terraces. Conglomerates as a source of diamond production may not be very significant in economic terms but are extremely important in geological terms because they are the main, known sources of the recent alluvial deposits of Brazil. In this sense, Brazil shows a sharp contrast with other large-scale producing countries, such as South Africa, Botswana, Russia and Australia, where the production comes largely from kimberlites. It has similarities with Angola and India. In the former, the Calonda conglomerates and, in the latter, the Banganapalli conglomerates are as important as sources as are the kimberlites, either because diamonds are recovered from them directly or because they feed the alluvials. But Brazil differs from these countries regarding the age of the oldest diamond finds. While in Angola they are restricted to the Cretaceous and in India to the Upper Proterozoic, in Brazil they spread from the Lower/Middle Proterozoic to the Cretaceous. Brazil is thus a very privileged area to study diamond sedimentary deposits, not only because they are numerous and have economic importance, but also and mainly because they occur in sedimentary sequences of at least four different ages. The recognition of common geological features in diamond sedimentary sequences of different ages led to the conclusion that their recurrence through time reflect fundamental processes which can be put together in a rift evolutionary model. In this study seven different diamondiferous districts in Brazil are briefly described and their rift-related features are emphasised. These features include the position in the sedimentary pile both of the diamond-bearing conglomerates and the volcanic events, as well as the proximal character in relation to growth-faults and the horizontal linear distribution of the conglomerates. The proposed model, stems from a sedimentary reasoning. It links primary diamond sources and related volcanism to rifts, and substantiates the idea that kimberlite/lamproite extrusions predate tectonic paroxysm such that, after erosion, diamonds from those extrusions end up in proximal rift-infill sediments. Here the timing of the extrusion of the primary diamond source rocks is similar to the one determined by White et al. who produced structural evidence to establish their model. Received: 13 January 1997 / Accepted: 22 July 1997     

Xenoliths of Eclogites with Diamonds from the Yubileinaya Kimberlite Pipe,Yakutia

The first results of study of minerals and diamonds of diamond-bearing eclogites from kimberlites of the Yubileinaya pipe with a variable percent amount of clinopyroxene and garnet are presented. Samples with a garnet content from 30 to 90% of the xenolith volume are dominant among the round to oval xenoliths with diamonds. Five eclogite samples contain grains of accessory rutile, as well as corundum and kyanite. Some samples host two or more diamond crystals.     

Carbon isotopic composition of diamonds in Ukraine and their probable polygenetic nature

The paper presents newly obtained data on the mineralogical and geochemical characteristics of Ukrainian endogenic and supergenic diamonds, including pioneering data on the carbon isotopic composition of individual diamond crystals. The endogenic diamonds occur as euhedral microcrystals (their morphology varies from octahedral to rhombododecahedral and cubic) with broadly varying concentrations of the structural nitrogen admixture and with mostly low aggregation of nitrogen centers. According to their carbon isotopic composition, Ukrainian diamonds range from kimberlitic–lamproitic to metamorphic and even carbonado-like, i.e., are polygenetic. Our data confirm the earlier conclusion that the supergenic diamonds are of impact genesis.     

津巴布韦金刚石独特的形态及其“指纹”特征的意义

津巴布韦马朗金刚石矿床为近年来发现的特大型高品位金刚石砂矿。对津巴布韦马朗金刚石产区的金刚石样品进行了矿物学和宝石学研究。结果显示,津巴布韦金刚石的晶体形态、表面微细特征等与世界上主要产地的金刚石有较大差异。其中最新发现的“十字架”形熔蚀图像和红色斑点是该产地金刚石的典型特征。“十字架”是由大量大小不等的正方形熔蚀坑沿晶体[100]重叠排列而成的,红色斑点为铁质氧化物沿熔蚀坑棱线生长而成的。研究结果表明,津巴布韦马朗金刚石的形态特征具有产地“指纹特征”的意义,对中国地勘单位赴境外进行金刚石找矿具有借鉴意义。     

贵州镇远地区含金刚石母岩再认识

贵州镇远马坪"东方一号"岩体为中国最早发现的原生金刚石矿,受当时只有金伯利岩才含金刚石矿及后来西澳阿盖尔钾镁煌斑岩型金刚石原生矿等的影响,先后定名为金伯利岩、钾镁煌斑岩、金云火山岩等。最近专题调查分析研究表明,镇远马坪地区含金刚石母岩,其岩石学矿物学和地球化学特征均更接近澳大利亚典型金伯利岩,白坟地区岩体则类似于澳大利亚典型钾镁煌斑岩,建议将镇远马坪地区"东方一号"等岩类定名为角砾凝灰质金伯利岩,白坟地区岩类定名为钾碱镁闪石-透辉石-金云母钾镁煌斑岩,镇远地区兼有金伯利岩和钾镁煌斑岩的特征,与西澳大利亚极其类似,镇远地区乃至黔东地区具有较大的金刚石原生矿找矿勘查潜力和研究意义。     

Re–Os systematics of diamond-bearing eclogites from the Newlands kimberlite

A suite of 14 diamond-bearing and 3 diamond-free eclogite xenoliths from the Newlands kimberlite, South Africa, have been studied using the Re–Os isotopic system to provide constraints on the age and possible protoliths of eclogites and diamonds. Re concentrations in diamond-bearing eclogites are variable (0.03–1.34 ppb), while Os concentrations show a much more limited range (0.26–0.59 ppb). The three diamond-free eclogites have Re and Os concentrations that are at the extremes of the range of their diamond-bearing counterparts. ¹⁸⁷Os/¹⁸⁸Os ranges from 0.1579 to 1.4877, while ¹⁸⁷Re/¹⁸⁸Os varies from 0.54 to 26.2 in the diamond-bearing eclogites. The highly radiogenic Os in the diamond-bearing eclogites (γ_Os=23–1056) is consistent with their high ¹⁸⁷Re/¹⁸⁸Os and requires long-term isolation from the convecting mantle. Re–Os model ages for 9 out of 14 diamond-bearing samples lie between 3.08 and 4.54 Ga, in agreement with FTIR spectra of Newlands diamonds that show nitrogen aggregation states consistent with diamond formation in the Archean. Re–Os isochron systematics for the Newlands samples do not define a precise isochron relationship, but lines drawn between subsets of the data provide ages ranging from 2.9 to 4.1 Ga, all of which are suggestive of formation in the Archean. The Re–Os systematics combined with mineral chemistry and stable isotopic composition of the diamond-bearing eclogites are consistent with a protolith that has interacted with surficial environments. Therefore, the favored model for the origin of the Newlands diamond-bearing eclogites is via subduction. The most likely precursors for the Kaapvaal eclogites include komatiitic ocean ridge products or primitive portions of oceanic plateaus or ocean islands.     

Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster,Botswana

This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of only 100–200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows: up to 10–20 m cover at 20–50 l, 20–30 m cover at 50–100 l and 30–80 m cover at 250 l. It is important to appreciate that the discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.

     

Middle Paleozoic kimberlite magmatism in the northeastern Siberian craton

The mineral chemistry and crystal morphology of kimberlite pyropes from the Billyakh River placer in the northeastern Siberian craton are characterised in terms of the placer history. The pyropes bear signatures of chemical weathering (dissolution), presumably in a Middle Paleozoic laterite profile, and therefore were originally hosted by Middle Paleozoic kimberlites. The broad occurrence of placer pyropes with lateritic dissolution signatures points to the presence of Middle Paleozoic diamond-bearing kimberlites in the study area.     

Giant swarms of Precambrian mafic dikes and potential diamond resources of ancient platforms

By the examples of the Siberian Platform and Canadian Shield, it is shown that spatial juxtaposition of Phanerozoic diamond-bearing kimberlite fields with giant swarms of Precambrian mafic dikes is caused by both systematic and incidental events. The first of these include (1) origination of mantle plumes and associated lenses of high-temperature mantle melting in the subequatorial “hot belt” of the early Earth, (2) formation of magma chambers that generated mafic dikes in these asthenospheric lenses, (3) shear stress, and (4) ultrahigh-pressure metamorphism of igneous and country rocks. As a result, the association of diamond-bearing high-density mafic and ultramafic rocks was formed under favorable thermal and fluidal conditions. These processes occurred first in the embryonic (multiplate) Neoarchean tectonic setting at a depth of 40–60 km (present-day elevation marks) and then at a deeper (100–150 km) level during the transition to the Proterozoic true plate tectonics. These processes left behind giant swarms of Precambrian mafic dikes, as well as structurally and genetically related deep-seated morphological and density anomalies. The relatively high position of two lithospheric units of diamond-bearing rocks, each underlain by a thick layer of the cold mantle, prevented these units from thermal and mechanical erosion during subsequent plate-tectonic stages characterized by deeper location of asthenospheric layers. The occurrence of clusters of Phanerozoic diatremes in ancient giant swarms of mafic dikes, as well as the enrichment of pipes in xenogenic diamond-bearing material derived from different levels of the tectonically delaminated lithosphere, may be attributed to incidental events that controlled the fertility of a relatively small number of kimberlite pipes.     

Diamonds from the Poiskovaya,Zapolyarnaya, and Leningrad kimberlite pipes,Northern Yakutia: Correlation of carbon isotopic composition and nitrogen content as an indicator of fluid diamond formation

Compared to most studied kimberlite pipes of the Yakutian province, diamonds from the Poiskovaya, Zapolyarnaya, and Leningrad pipes (about 100 determinations) are characterized by a decrease in the average δ¹³C (−4.59, −4.50,−4.04‰) and by relatively low average nitrogen contents (93, 254, 304 ppm, respectively). These pipes also differ in the relative abundance of alpha-1 group crystals in Galimov’s classification. Based on a stable combination of morphology and properties, we distinguished diamond populations of the common origin. All of them have significant a negative correlation between δ¹³C and the nitrogen content. This correlation is considered to be a result of synchronous and progressive loss of nitrogen and the light isotope ¹²C in the medium during the diamond formation. The conclusion is drawn that the alpha-1 crystals were formed in the open fluid system. The pyrolysis of ethane—C₂H₆ → CH₄ + H₂ + C_diam—is assumed to be a model of diamond precipitation from fluid. The pressure release due to extension deformation of the mantle under tectonic action might be a factor shifting this reaction toward diamond formation.     

Physicochemical parameters of the material of mantle plumes: Evidence from the thermodynamic analysis of mineral inclusions in sublithospheric diamond

Thermodynamic analysis of equilibria involving minerals of the lower mantle of pyrolite composition and crystalline carbon-bearing compounds indicates that the range of oxygen fugacity values at which diamond can be formed is separated from the region in which Fe-rich metallic alloy is generated by a field in which Fe carbides are stable. This implies that diamond can be formed in the lower mantle under more oxidizing conditions than those thought to be dominant in this geosphere. The absence of a metallic phase from the lower-mantle diamond-bearing mineral assemblage is consistent with the high (approximately 1%) Ni concentration in the ferropericlase found as inclusions in diamonds (Fe-rich metallic alloy is able to intensely extract Ni). An elevated redox potential also follows from the occurrence of carbonate phases found among mineral inclusions in lower-mantle diamonds. The main reason for a local increase in oxygen fugacity in the lower mantle may be shifts of redox equilibria toward a decrease in the amount, and then the disappearance of the Fe-Ni alloy with increasing temperature. An important role in the formation of diamond may be played by the generation of carbonate-phosphate and silicate melts in high-temperature zones and the migration of these melts and their interaction with wall rocks.     

Fe–S melt as a likely solvent of diamond under mantle conditions

The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P–T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.     

Metal films on the surfaces and within diamond crystals from Arkhangelskaya and Yakutian diamond provinces

Representative samples of diamonds from five kimberlite pipes (Lomonosovskaya, Archangel’sk, Snegurochka, XXIII Congress of the Communist Party of the Soviet Union (CPSU), and Internationalnaya) of the Arkhangelskaya and Yakutian diamond provinces in Russia have been studied. Thirty-three varieties of metal films have been identified as syngenetic associated minerals. The films consist of 15 chemical elements that occur in the form of native metals and their natural alloys. Remnants of metal films were detected within diamond crystals. The metal films coating diamonds are a worldwide phenomenon. To date, these films have been described from Europe, Asia, South America, and Africa. Native metals, their alloys, and intermetallides are actual companion minerals of diamond.     

Comparative characteristic of diamonds with olivine inclusions from the Ebelyakh placer and kimberlite pipes of the Yakutian Diamondiferous Province

The results of morphological examination and the character of the structural orientation and estimation of residual pressure calculated from spectra of combination dispersion in olivine inclusions within diamonds of the Ebelyakh placer and kimberlite pipes of the Yakutian Diamondiferous Province are presented. The data analysis aimed at revealing indications of similarity and/or differences between diamonds from the pipes and the placer. Differences in the structural orientation and spectra of combination dispersion of the inclusions of olivine in dodecahedroids of placers of the northeastern part of the Siberian Platform support the assumption of their non-kimberlite nature.